The present invention is directed to a process and an apparatus for gravure using an erasable and reusable gravure form proceeding from a gravure blank form with a base screen which is designed at least for the maximum amount of ink to be transferred.
Gravure refers to a printing process using printing elements which are depressed relative to the surface of the form. After the printing form is completely inked, the printing ink is removed from the surface. The ink remains only in the depressed areas. Copper-coated steel cylinders, hollow cylinders mounted on tensioning cores or, in many cases, copper plates clamped on cylinders may be used as printing forms, for example.
Due to the type of inking and the wiping of the surface of the form with doctor blades, pure surface printing is not possible. The entire graphic must be resolved into lines, dots or screen elements. Due to their differing depth and magnitude, the individual printing elements take up varying amounts of printing ink. Consequently, the impression will have different ink values at different locations on the image.
Various working methods are currently used for producing a gravure form. For instance, in the variable-depth method, the etching principle consists in a gradual diffusion of concentrated ferric chloride solutions through a pigment-gelatin layer. The pigment reproduction on the copper printing form is formed of a hardened gelatin relief corresponding to the gradation of tones of the transparencies. The engraving process is characterized by line-scanning of the image and text by photocells and simultaneous engraving of the printing form by engraving heads. It should be noted in particular that depressions are made in the copper layer of the printing form by means of a high-energy electron beam which is directed on the blank form under vacuum and removes material in conformity with the intended image. The printing form which is engraved in this way can be provided with screens with varying depths and surfaces.
Depressions can also be made using a high-energy laser beam. In so doing, appropriate steps must be taken to ensure that the laser energy is coupled to the substrate, since copper is especially prone to reflect a laser beam when not subjected to special preconditioning.
Further, DE-OS 27 48 062 discloses a process for producing an engraved printing form in which a gravure blank form is first prepared by providing the smooth surface with depressions of equal depth and magnitude in a uniform manner and then covering the engraved surface with a light-sensitive substance so as to fill up all of the depressions. The blank form is then exposed photographically with the desired image so that the exposed areas are polymerized and the unexposed portions can be washed off, resulting in a differentiated image.
It can be asserted in general for all gravure processes that the depth of image locations on the printing form is greater than that of nonimage locations. In doctor-blade gravure, in particular, the screen grid forms webs of uniform height which define the image locations and form a support surface for the doctor blade. A special set of printing form cylinders (for each printing ink there must be one printing form cylinder with a corresponding number of printing sides) is required for every printing job. These cylinders are produced with the required cylinder circumference depending on the printing format. When setting up the gravure press or rotary printing machine, the appropriate printing form cylinders must be exchanged. A modern cylinder of this type, e.g., with a width of 200 cm, weighs approximately 800 kg. The mechanical cost for the processes described above is very high, since these processes can only be carried out outside the printing machine. In addition, each of these production processes involves steps such as electroplating or coating, exposure and development, which rules out the possibility of reusing the same printing form without extensive processing, in particular chemical processing. Further, after etching or engraving to form the image, that is, after removal of material, chroming is usually carried out to prolong service life.
If the printing form is to be stored for subsequent repeated applications, it is generally necessary to reserve space for the entire cylinder. For this reason, production of printing forms is very involved and therefore expensive, particularly when electroplating is required. Moreover, the resulting toxic sludge is objectionable in ecological respects.
On the other hand, DE 38 37 941 C2 which corresponds to U.S. Pat. No. 5,072,671 discloses a process for producing a gravure form in which the image can be produced directly in the printing machine and in which, moreover, the image can be removed from the gravure form in the printing machine and the gravure form can be prepared for a new image. Likewise in this case, a gravure blank form is produced with a base screen designed at least for the maximum amount of ink to be transferred. In the printing machine, an amount of thermoplastic substance in inverse proportion to the image information is then introduced into the depressed portions from a nozzle of the image-point transfer unit or by means of image-correlated ironing so as to reduce the effective volume of the depressions. In other words, in contrast to the other methods, the image is formed on the gravure blank form by image-forming application of material. After the printing job, the thermoplastic substance can then be liquified in the printing machine by means of a heat source and removed from the printing form cylinder by a wiping and/or blowing or suction device. This prior art teaches computer hardware and software which is useable in carrying out the present invention. As such, rather than repeat the subject matter, applicants incorporate the subject matter of U.S. Pat. No. 5,072,671, herein by reference.
However, the application of material to form images raises problems with respect to the positioning accuracy of the image. Material deposited on the webs cannot easily be introduced into the depressions completely. Yet, in order for all of the transferred material to contribute in a desired manner to the reduction in the effective volume of the depressions, this material must be introduced in its entirety.
Accordingly, the object of the present invention is to develop a process and an apparatus for gravure printing in which the gravure printing form can be produced inexpensively and also directly in the printing machine and in which the positioning of the image can be made more accurate.
Storage of gravure forms is eliminated since the cycle of characterizing process steps can be carried out repeatedly.
Another special advantage of the process according to invention and of the apparatus for carrying out this process consists in that wear on the gravure blank form is compensated for because the maximum image-forming depth in the applied substance on the gravure printing form is appreciably less than the original depth of the depressions of the prestructured blank form. That is, if the depth of the depressions is reduced due to wear on the webs, the maximum image-forming depth can nevertheless be achieved by a wide margin. For this reason, the webs of the blank form are also advantageously constructed so as to extend vertically to the surface of the gravure form as far as possible.
Advantageous constructions are contained in the dependent claims.